In a typical method of manufacturing silica glass used for optical fibers, referred as just optical fiber base material, generally using the OVD process, raw materials for glass are flame-hydrolyzed in oxyhydrogen flame, generated glass fine particles are deposited on a rotating target rod to make a major part of porous glass base material, and then the porous glass base material is dehydrated and transformed into clear glass.
As the optical fiber market has been faltering recently, lower-cost optical fibers are demanded. Reducing the manufacturing cost is one of the most important issues in manufacturing optical fiber base material as precursor of optical fibers.
The glass fine particles generated by flame hydrolysis are carried in flame flow, blown against and deposited on the porous glass base material, but a significant amount, almost the half is carried away in effluent gas to the outside of the system and disposed of without being deposited. The method is required, in which the glass fine particles generated from supplied raw materials for glass is deposited on the porous glass base material with higher efficiency.
To deposit the glass fine particles with high efficiency in the above OVD process, it is required to examine the following factors in detail: the relative rate and distance between the target rod as the starting base material and the burner used for depositing glass fine particles; the structure of the burner used for deposition; and the flow rate of gas supplied to the depositing burner.
If you seek the optimal condition for each of the factors, it takes a lot of time and money. Despite you could get the optimal condition for each factor, when each factor changes, you should reexamine that.
Patent Document 1 discloses that using a concentric multi tube depositing burner, when glass fine particles generated by flame-hydrolyzing raw materials for glass are deposited on the target, Reynolds value Re of the channel of raw material in the depositing burner is controlled depending on the diameter of the glowing target during deposition so that the deposition efficiency can be improved.
To improve the deposition efficiency in this method, however, the Re value should be decreased. It is difficult, however, to keep the deposition rate high as improving the deposition efficiency.
It has been known to solve the problem easily that the surface of soot is forcedly cooled down.
The following may be a reason why the deposition rate is improved when the porous glass base material is forced to be cooled. There may cause a heat migration phenomenon, which may be the basic mechanism in depositing the glass fine particles generated by the depositing burner on the surface of the target. The above phenomenon can explain that when there exist particles in the atmosphere, and the temperature of ambient gas has a gradient, the particles migrate from the high temperature area to the low temperature area.
In the OVD process, the surface of the porous glass base material is forced to be cooled, except for the area where the particles which are generated by flame hydrolysis, blown by the relatively moving depositing burner, and carried in flame to directly hit the target. When the depositing burner returns to the cooled area, the temperature gradient increases as compared the cooled area with the neighboring area around the surface of the base material which has a high temperature due to the flame during the deposition. This causes the glass fine particles to migrate to the low temperature area so that the deposition efficiency is improved.
There are some prior arts like below disclosing methods of cooling the deposition surface of glass base material.
Patent Document 2 discloses the method in which an outlet of cooling gas is disposed against the deposition surface and immediately above the depositing burner, and nitrogen gas N2 or argon gas Ar is blown against the deposition surface to cool the porous glass base material. Patent Document 3 discloses the method in which helium gas He as a cooling gas is blown against the starting base material to cool to the surface temperature of about 500 degrees, and then the deposition starts to form porous glass layer. Patent Document 4 discloses the method in which an outlet of cooling gas is disposed against the deposition surface and immediately above the depositing burner, ion jet flow generated by corona discharge cools the surface of the porous glass base material during the deposition. Patent Document 5 discloses the method in which a part of the surface of the rotating target where the flame isn't touching is forced to be cooled by the water jet flow from a water cooled nozzle.    Patent Document 1: The Japanese laid-open patent No. 2001-294429    Patent Document 2: Japanese laid-open patent No. 1986-86440    Patent Document 3: Japanese laid-open patent No. 1989-203238    Patent Document 4: Japanese laid-open patent No. 1989-65040    Patent Document 5: Japanese laid-open patent No. 1992-55336